Such a method is known from U.S. Pat. No. 5,027,281, wherein a dental prosthesis is made from a massive block of material. Material is thereby removed from the block of material by means of a numerically controlled micro milling machine. After the shape of the prosthesis has been determined and stored in the memory of a computer, the computer calculates the machining paths which the miller is to follow. The machining paths are determined by means of flat, mutually parallel sections of the intended prosthesis. Consequently the machining path which the milling tool follows during the material removing operation lies in a flat plane at all times. In order to remove the machining paths which the final material removing operation leaves behind, the surface is polished, for example by means of a rotating brush.
U.S. Pat. No. 4,937,928 describes a method for manufacturing a dental prosthesis, wherein the prosthesis is formed on a model in the shape of the part of the teeth on which the prosthesis is to be provided. The prosthesis is thereby produced by successively applying a number of layers of material on the model. After the application of each layer the workpiece is worked by means of a numerically controlled machine tool. The machining paths which the tool follows during this operation are computed by means of a CAD/CAM system.
A drawback of the known method is that either the machining paths must be spaced so closely that the individual paths on the final product are no longer visible, which is very laborious, or the workpiece must be subjected to an intensive polishing or grinding operation, in order to obtain an aesthetically sound result. Such a polishing operation is usually carried out by hand.
The material which is used for a dental prosthesis must satisfy a number of varying requirements. Besides requirements relating to the color and machinability, the material must be sufficiently hard, which generally entails a certain degree of brittleness. This is particularly so when use is made of materials such as porcelain or glass especially suitable for mechanical dentistry because of their machinability. In order to obtain sufficient strength, in spite of brittleness, it is not unusual to provide the inner side the dental prosthesis, which is not visible once the prosthesis is fitted, with a metal layer. This metal layer may even comprise a thickness of a few tenths of a millimeter and extend across the entire inner surface of the prosthesis.
The dental prosthesis is fitted on the dental element in question with the interposition of a cement layer. The cement layer generally comprises a thickness of up to about 0.3 mm and is sufficiently strong for transmitting the forces which are exerted on the prosthesis to the dental element in question, especially during chewing.
When the dental prosthesis substantially consists of a relatively brittle material, the most vulnerable place for a fracture to occur is in the area near the edge of the prosthesis, i.e. in the area where the prosthesis joins the teeth. Since the part of the teeth in question is prepared --in advance--up to the place where the edge of the prosthesis joins the teeth. Where the edge of the prosthesis joins the teeth is called the preparation line, which, in case of a dental crown, generally extends around the dental element in question, near the part of the jaw where the dental element is implanted. The edge of the prosthesis therefore corresponds with the preparation line of the dental element in question.
In order to obtain a good connection of the prosthesis to the part of the teeth in question at the location of the preparation line, the prosthesis comprises a relatively thin part near the edge. When a reinforcement in the form of a metal layer is used, a metal layer is provided especially in that area, in order to strengthen this area.